1. Field of the Invention
The present invention relates to a sheet supply apparatus incorporated into an image forming apparatus such as a printer, a copying machine, a facsimile and the like, and more particularly, it relates to a sheet supply apparatus having a lifter means for maintaining a height of an uppermost sheet substantially constant.
2. Related Background Art
Among sheet supply apparatuses incorporated into an image forming apparatus, there is a sheet supply apparatus having a lifter means for maintaining a height of an uppermost sheet in a sheet stack substantially constant so that a sheet is fed out by a sheet supply means such as a sheet supply roller at a constant height position. In such a lifter means, in order to maintain the height of the uppermost sheet constant, for example, a lever is abutted against an upper surface of the sheet stack, displacement of the lever is detected by an electrical sensor such as a photo-sensor, and an electrical actuator such as a motor or a strong solenoid for lifting and lowering a sheet stacking plate on which sheets are stacked as the sheet stack is energized on the basis of a detection result of the electrical sensor.
Further, in some conventional sheet supply apparatuses, a sheet supply roller can be shifted to be abutted against or separated from the uppermost sheet of the sheet stack the height of which is kept constant by the lifter means. In this arrangement, after the sheet is fed out by the sheet supply roller abutted against the sheet, by separating the sheet supply roller from the sheet, load (back tension) acting on the sheet is reduced.
In the past, an exclusive electrical actuator such as a motor or a solenoid similar to that associated with the lifter means has been used as a mechanism for lifting and lowering the sheet supply roller with respect to the sheet. However, in the conventional sheet supply apparatus having the lifter means and the mechanism for shifting the sheet supply roller, the following problems arose:
(1) Since the electrical sensor, the actuator for the lifter means and the exclusive actuator for lifting and lowering the sheet supply roller must be provided, the entire sheet supply apparatus becomes expensive, and large electrical capacity is required;
(2) Since complex wiring is required for electrically connecting the electrical sensor and the exclusive actuators to associated elements, miss-wiring is apt to occur; and
(3) Electrical control sequence for the sheet supply system becomes complex, and, thus malfunction is apt to occur.